The present invention relates to a ceramic-metal joint structure.
Joining ceramics to metals is difficult because the great thermal-expansion mismatch between ceramic and metal causes residual stresses to develop from thermal strain, leading to frequent failure in the ceramic part. Two techniques are currently employed in bonding ceramics to metals which do not cause failure of the ceramic; one is to use materials having similar thermal expansion coefficients such as alumina and Kovar (the trademark for a Co--Ni--Fe alloy) and the other method is to use a metal insert having a thermal expansion coefficient close to that of the ceramic.
These methods, however, are not suitable for use with ceramics such as silicon nitride having small thermal expansion coefficients. Tungsten is one of the metals having low thermal expansion coefficients but because of its high price and brittleness as well as its easily oxidizable nature, tungsten is not at all suitable for use as a mating metal or a metal insert with the small-expansion ceramic that could be used in those parts around an internal combustion engine which are exposed to high temperature and constant vibrations of high magnitude. It has therefore been desired to directly bond the low thermal expansion ceramics to ferrous or aluminum alloys.
In another method that is known in the art of ceramic-metal bonding, a mixture of ceramic and metal that exhibits a gradual change in thermal expansion coefficient is thermal-sprayed onto the mating surface of the ceramic, which is subsequently bonded to the metal by friction, compression or other suitable techniques. This method however is not highly efficient since it is quite difficult to produce a thermal-sprayed coat having a continuous profile of thermal expansion changes.